Recirculation system for steam generator



June 1964 w. w. SCHROEDTER 3,135,252

RECIRCULATION SYSTEM FOR STEAM GENERATOR Filed July 27, 1961 5 Sheets-Sheet l INVEN'TQR W- SCHROEDTER ATTORNEY W ILLBURT F g I June 2, 1964 Filed July 2'7,

FIG-8 w. w. SCHROEDT'ER RECIRCULATION SYSTEM FOR STEAM GENERATOR 1961 5 Sheets-Sheet 4 INVENTOR WILLBLJRT W. SCHROEDTER ATTORNEY conditions at all loads.

turbine throttle valve.

United States Patent 3,135,252 RECRCULATION SYSTEM FOR STEAM GENERATOR Willhurt W. Schroedter, West Hartford, Conn., assignor to .(Iomhustion Engineering, Inc, Windsor, Qoun, a corporation of Delaware Filed July 27, 196i, Ser- No. 127,395 Zil Claims. (Cl. 122-406) This invention relates to steam generators and particularly to the recirculating systems of once-through, supercritical steam generators.

An object of this invention is an improved recirculating system for a once-through steam generator for operation in the supercritical pressure region.

' In the operation of steam generators, custom and experience has dictated that a minimum rate of circulation of the water or working mediumin the heating tubes must be maintained at all times while there is fire in the furnacedn order to prevent the burning of the tubes forming the walls of the furnace, commonly known as the water walls, and containing the working medium, usually purified water. Custom and experience has indicated that an average velocity of about 3 feet per second for example at the entrance to the water wells is the 'minimum safe velocity at all operating conditions of the generator to provide the turbulent flow required to prevent localized heating and burning of the tubes. This velocity is a criteria which determines many of the other constructional features of the steam generator.

As indicated above, the object of circulation is to assure continuous removal of the heat absorbed so as to i keep the tubes'from exceeding their designed temperatures. This circulation must produce therequired fluid flow in the tubes at the most economical balance between tubing cost and available or required pressure drop and also provide proper distribution of the fluid among multiple tube circuits with suflicient stability against transient Modern steam generators, regardless of pressure level, have by'their very nature, (1) a water heating section or economizer, (2) an evaporator or at supercritical pressures,'an intermediate section,

which by convention is commonly termed furnace walls, and finally (3 a superheater. The pressure level or working pressure at which the steam generator operates influences particularly the relative size of these sections and the temperature level.

. The flow in economizers or the water heating section and in superheaters in all steam generators is maintained by the feed water pump which for any load balances the pressure drop of the fluid from the pump outlet to the The basic difference in steam generators in regard to circulation is restricted to the circulation of water in the evaporator or furnace walls.

Three methods of circulation have been employed,

namely, natural, controlled, and once-through. .The first.

two methods may also be characterized as using recirculation. V

a In natural circulation water is heated and boils in the heat-absorbing riser tubes and the mixture of water and steam rises upward into a drum located above the riser tubes where the steam separates from the water and flows on into the superheater. The incoming feed water is fed into the drum and mixes withithe water in the drum and is returned through a downcomer to the lower end of the heat absorbing riser tubes. Circulation is by a thermal pump deriving its power from the heat absorbed and 7 depends upon the large change in volume between saturated water and steam at sub-critical pressures andmust provide a substantial ratio of saturated'water to steam in order to assure adequate cooling of all evaporator tubes.

Becauseof the limited thermal pumping head and ,the

3 ,l35,252 Patented June 2, 1964 the separating drum and the bottom of the heat absorbing riser tube or water wall in a structure otherwise similar to the natural circulation structure described above. The motor driven pump, which is now added to the thermal pump, can provide a much larger head and the volume of circulated water is reduced so that smaller tubes of 1 /2" in diameter can be used. Orifices at the inlet header to the water walls are used to distribute water to the parallel circuits of diiferent resistance in proportion to their heat absorption. The motor driven pump must be continuously operated, in addition to the feed water pump, which does not provide any of the circulation, in order to provide adequate circulation at all loads. In practice this pump pumps about four times the weight of water at full load as the weight of steam that isbeing utilized by the turbine. pumps the same large amount of water at all loads, which is usually the case, the proportion of water pumped to the steam used is accordingly much greater at low loads. Both the natural and the controlled circulation method recirculate fluid through the evaporator or furnace wall section. Both of these methods require a drum for separating the recirculated fluid, in the form of water, from i the steam passing through the system to the turbine. The

feed Water pump only supplies feed water to the drum and pressure to the system but does not participate in the circulation of fluid through the furnace wall.

, If we eliminate the separatordrum and the recirculation through the evaporator associated therewith and connect the feed pump directly to the bottom of the heat absorbing riser tube or furnace wall we have a pure once-through circulation. The output flow now passes in a through flow passageway or circuit in sequence through conduit means including the economizer, the evaporator or furnace wall, and the various sections of the superheater. A drum for separation of water and steam is not needed so that a unit of this kind may be operated at subcritical and at supercritical pressures as well. In the simple once-through system just described the only circulation in the water wall is through-flow provided by the feed water pump forcing the working fluid in the through-flow path or passageway through the entire system to the point of use. This presents a problem because the flow in the furnace wall must not at any time be less than the minimum safe velocity. If this flow is established at say 30% of the full load of the system, then' the feed water pump must never deliver less than 30% of its full load capacity. In starting up the system this will require that this quantity of water amounting to 30% of the full load of the system must be heated in the evaporator and superheater tubes and then. condensed in the condenser during the time that the boiler is being started up and before the turbine is started. All this heat is discharged as waste heat in the condenser cooling water. As the starting up time may take several hours,

including the time at which the turbine may be run at evaporator section. This is shown in .British Patent If the motor is uncontrolled and 3 768,201 of Babcock and Wilcox, Ltd. published Feb. 13, 1957, in which the circulating pump for recirculating the fluid through the evaporator has a maximum head limited substantially to the pressure drop which would be produced when the vapor generating and superheating unit is operating at the lowest load permissible without circulation from the pump.

Another method of controlling the recirculation through the water Walls of a once-through steam generator is shown in British Patent 831,175 of Sulzer, Freres, Societe, Anonyme, published Mar. 23, 1960, and Armacost US. Patent No. 3,038,453 for Apparatus and Method for Controlling a Forced Flow Once-Through Steam Generator issued June 12, 1962, in which the recirculating pump is provided with a special control device for regulating the pump. The control device may be responsive to any of several variables which are dependent upon the load, for instance, on the pressure drop occurring within a predetermined section of the fluid heating device, to variations, in the speed of the feed pump, or to the electrical output of a turbo-generator, the turbine of which is supplied with steam from the steam generator. The regulating device may take several forms, for instance a throttle valve in the recirculating, system controlled by the control device or, in the case where the recirculating pump is driven by a variable speed motor, it may comprise speed adjusting means for this motor controlled. by the control device. This publication indicates also that this particular recirculating system can be applied to steam generators operating in the supercritical pressure range.

The present invention relates to a once through steam generator in which the through-flow consists of the feedwater and its vapor which continuously passes through the generator to the point of use. A recirculation system having a recirculation passageway including a selected portion of the generator, such as the water walls, is superimposed on' the through-flow and provides recirculation through the selected portion of the generator in addition to the through-flow and is particularly adapted for operation in the supercritical pressure range and is an improvement over any of the devices shown in either of the above mentioned British publications. In the present invention, control and eventual elimination of the recirculatien in the supercritical system is automatically obtained without the head limiting means of the one publication or the control and regulating devices of the other publication. The heat and pressure characteristics peculiar to the supercritical operation are ultilized to automatically balance the output of a free-running, uncontrolled centrifugal pump in the recirculating system to maintain a sufiicient but limited recirculation flow at partial power conditions and where desired to automatically block the recirculation flow at load conditions above a preselected amount.

It "should be noted here that in the supercritical process of heat addition the single phase fluid continuously changes temperature in its transition from water heating to steam superheating and at no single point in this type of steam generation are there fluids with simultaneously differing densities such asexists when the latent heat of evaporation is being added at subcritical conditions. Thus the supercritical process rules out the use of a water separator such as the separator drums employed in the subcritical units. It should also be pointed out that because of the single phase nature of the fluid it is" possible to .pump it at any temperature as long as the pressure remains in the supercritical range which is a marked distinction from operation in the subcritical range where it is not practical to pump a mixture of water and steam.

With the above background in mind, other features and advantages will be apparent from the specification and claims and from the accompanying drawings which will illustrate an embodiment of the invention which will be described by way of example and in which:

FIGURE 1 is a diagrammatic representation of forced flow once-through steam generating power plant embodying the invention;

FIGURE 2 is a diagrammatic representation of a steam generator of the forced flow once-through type operating in the supercritical pressure range, and having in accordance with the invention a recirculation circuit superimposed upon the water heating tubes, the flow therethrough being automatically controlled by a constant speed, uncontrolled recirculating pump floating on the once-through flow circuit; the gas duct containing the superheater has been moved from the rear to the side of the furnace, and the economizer has been omitted for clarity;

FIGURE 3 is a graph showing the relation between temperature and specific volume (cubic feet per pound) in a fluid (water and steam) maintained at supercritical and subcritical pressure;

FIGURE 4 is a graph showing the fluid temperatures in the furnace walls including the center Walls and water walls of a steam generator operating at supercritical pressures;

FIGURE 5 is a graph of the pump characteristics and the system curves forthe system of FIG. 2;

FIGURE 6 is a graph of the recirculation flow for various percentages of the total feed water flow;

FIGURE 7 is a graph showing the water wall velocities produced with the recirculating flow pump operating at various percentages of feed water flow;

FIGURE 8 shows the head produced by the recirculating pump in a graph similarto FIG. 7 but plotted in feet of Water being pumped at various percentages of feed Water flow; 7

FIGURE 9 shows a modification of the structure shown in FIG. 2, in which the recirculating pump location has been changed from a position in the inlet to a position in the outlet of the mixing vessel 20;

FIGURE 10 is a graph similar to FIG. 5 but showing the pump characteristics and system curves for the mixed flow pump;

FIGURE 11 is a graph similar to FIG. 8 but showing the head in feet of water being pumped by the mixed flow recirculating pump;

FIGURE 12 shows a further-modification of the structure. shown in FIG. 2 in which the recirculating pump is located between the center and water walls; and

FIGURE 13 shows a still further modification of the structure shown in FIG. 2 in which the recirculating pump is inserted in the through flow line at the outlet of the water walls.

In FIGURE 1 there is illustrated in diagrammatic form'the basic circuit of a forced flow once-through steam generator power plant operating in the supercritical pressure range. A feed pump F takes water from a water tank W and delivers this water under pressure to an economizer E wherein the water is first heated. From economizer E the water flows through additional heating tubes H which serve as a lining for the walls of the furnace and also may constitute convection heating surface exposed to the combustion gases after they have left the furnace. In tubes H transition takes place from the liquid phase to the vapor phase of the Working fluid before the steam is conducted to superheater S wherein the steam is superheated *to a fi'nal temperature of say 1000 F. Upon leaving the superheater S the steam is conveyed to a point of use such as turbine T where the thermal energy of the steam is converted into mechanical energy for driving an electric generator 'G. The feed pump pressurizes the entire generator system from the entrance of economizer E to the usual control valve for turbine T with a pressure known as working pressure. This pressure varies from one end to the other of the'system due to friction and other loses incident to forcing the fluid through the system and is usually measured at the turbine control valve or the superheater discharge end of the system. The steam exhausted from the turbine T is condensed in condenser C and returned to water tank W by way of condensate pump P.

In the recirculating circuit pump R is arranged in parallel with the pump F and may take working fluid from the outlet of tubes H andreturn it to the inlet thereof. The invention will be described particularly in connection with the operation of this pump R in which fluid taken from the outlet of the heating tubes H which will be called the center walls and water walls and returned to a mixing chamber M at the entrance to'the furnacewalls where the recirculated fluid is mixed with the feed water fluid to provide a mixed fluid which is fed to thefurnace walls.

FIGURE 2 shows a specific embodiment of the invention in which the amount of working fluid recirculated through the heating tubes 12, forming a center wall in the furnace, and tubes 14, forming the water walls in the furnace, is automatically regulated by the pump 1% driven by the constant speed electric motor 16. The pump 1 takes fluid from the header 18, connecting the tops of the tubes forming the water walls 14, and delivers it to the mixing chamber 29, which also receives water from the feed pump 22. The chamber has a discharge connection 200 to the header 24 connecting the bottom of the tubesforming the center wall 12. The chamber 2% is normally located near the top of the steam generator in order to obtain the benefit of the pressure differential between the cold water in line 2% and the heated fluid'in the fluid passes to the header 24 and then tmough panels of parallel heating tubes, with the panels formed of tubes 12 and 14, being connected in series and forming radiant and/or convection heating surfaces exposed to the heat of the hot gases in the furnace. These tubes 12 and 14 may form water wall linings within the furnace chamber 32 as shown in more detail in my application for Furnace Wall Arrangement for Vapor Generator Serial No. 127,-

396 filed on July 27, 1961, now Patent No. 3,135,243 of June 2, 1964. After passing through the tubes 12 and 14 the fluid passes to outlet-header 18 and thence in one direction to the superheater inlet header 48 through superheater 50, outlet header 52, steam pipe 54, to a point of use, such as a turbo-generator comprising turbine 56 driving an electric generator 58. The working fluid passes in another direction from the header 18 to form a parallel circuit through the recirculating pipe 60 to the recirculating pump 10 arranged in parallel with the feed pump 22. The steam from the turbine 56 is exhausted into condenser 62, the condensate passing through condenser pump 64, the feed water heaters 66 back into water reservoir 42 by way of pipe line 68 thus completing the steam power cycle.

The amount of working fluid recirculated through the tubes 12 and 14 is controlled entirely by the free running,

uncontrolled pump 10 floating on the once through circuit,

and particularly, as shown in FIG. 2, the portion of the "once-through circuit formed by tubes 12 and 14, and is located in the line 60, connecting the outlet header 18 with the mixing tank 2%, and forming apart of the parallel I recirculating circuitconnecting the outlet header 18 of the water walls 14 with the inlet header 24 of the water walls 12. The centrifugal pump 10% is driven by a substantially constant speed mechanism shown as a constant speed electric motor 16 having an onand off-switch 84 in the power lines 86 for the purposeof starting and stopping the recirculating pump'10. The conduit 60 including conduit 20d. is provided with a check valve 88 preferably at the pump 7 outlet and if desired may be provided with a throttle the usual slight expansion and contraction with pressure and temperature changes.

In operation the fuel supply and the feed water supply of the steam generator may becontrolled by any conventional control system to provide fuel and feed water supply in accordance with load.

In the embodiment chosen to illustrate the invention the control system may be divided generally into two control systems. One system controls the-turbine output and pressure by heat input. The other system controls feedwater flow and steam temperature in response to heat ab sorption. Both systems are intermeshed by a load output signal to the feedwater control and by a steam temperature signal to the heat input.

The one system is controlled generally by the load regulating computer 741 which provides error signals from desired load, pressure and frequency, to regulate and maintain the proper required output over the full load range. Computer output signals are sent to the control system for feedwater 49, fuel and air 38 and turbine governor 72. The control action for fuel and air is accomplished by the computer 70, so that fuel input through line 39 is regulated by valve 38 to maintain the proper steam generator output.

The other system coordinates the various systems controlling the internal steam generating process. Feed water control is accomplished by a feed water valve 40 in-conjunction with a flow meter 74. Feed water valve 40 is adjusted in accordance with heat input which in turn is adjusted by load. Heat input is sensed by water wall temperature. In addition the feedwater controller receives a signal from the combustion control system to maintain the proper fuel and feed water relationship. The water wall temperature is also adjusted to maintain a proper ratio of injection to the superheater, for steam temperature control (not shown), to total feed water flow.

In explaining the self regulating feature of the recirculating'pump it should first be noted that it is a centrifugal pump in which the head of the fluid pumped varies inversely with the quantity of the fluid being pumped. It'

should also be noted that the pump is pumping very hot fluid received from the header 18 at the top of the water walls and is delivering it to a mixing chamber 20 where it is mixed with the relatively cool feed water to provide a cooler mixture which is fed through the center wall where some heat is added and the temperature is raised before the mixture is fed to the header 30 at the entrance to the water walls. As explained at the beginning of this application custom and experience has dictated that the velocity of flow from the header 30 and at the entrance to the water walls 14 should not fall below the minimum; safe rate of 'say three feet per second for any normal operating condition of the steam generator. This positure, as the flow area is substantially constant. However,

asthe temperature of the mixture leaving the header 30 at the water wall entranceand the temperature of the mixture leaving the header 18 at the water wall outlet and being pumped by the pump 10 diverge with increases in feed water flow, see FIG. 4, it is apparent that the gallons 7 pumped by the pump 10 will vary with respect to the gallons leaving the header 30 due to this temperature change alone, in addition to the variation in the discharge characteristics of the pump 10 due to the increase in head imposed by the increase in feed water flow through the furnace walls.

To further explain this relation of the pump head and flow with respect to the load demandsof the steam generator, -reference may first be made to the pump characteristics shown in FIGURE in which the line 9 2 indicates the relation between the head in feet of the water pumped and the volume or gallons per minute of the pump fluid delivered by the pump. This relation holds independent of temperature of the pumped fluid. As the temperature in the different parts of the system have a marked effect on the quantity of fluid necessary to be pumped by the pump in order to maintain a three foot per second velocity at the entrance to the water Walls 14, attention is nowdrawn to the graph in FIGURE 4 in which line 94 shows the temperature of the fluid at the water wall outlet (header 18) plotted against the percentage of load or feed water flow. It Willbe noted that this temperature increases from about 731 at 5% load to about 758 at 100% load. This is the temperature of the water that is being pumped by the recirculating pump 10. Line 96 indicates the temperature of the water leaving the center wall which would be the temperature of the water in the header 26 and as this header is connected by a 'downcomer 28 directly with the header 30 at the inlet of the water walls 14 this center wall outlet temperatureis also the water wall inlet temperature and is the temperature of the water which establishes the three feet per second flow at the water wall inlet. It will be noted that although the recirculating water temperature increases with load the water wall inlet temperature decreases with load. This is due to the addition of the cold feed water from the feed pump 22 to the mixing vessel 20. Line 08 represents the temperature of the water at the center wall inlet header 24 and shows how the water leaving the mixing vessel has its temperature reduced with increasing loads due to the increased quantity of feed water and the reduced quantity of recirculating water being fed to the mixing vessel. Line 100 is an indication of the temperature of the feed water being fed by the feed pump 22 to the mixing vessel 20 to be combined with the recirculating water whose temperature is indicated by the line 94. The above temperatures are established by the furnace and water wall designs and are a function of the load.

To further appreciate .the condition afiecting the pump operation attention is called to FIGURE 3 in which the line 102 represents the changes in specific volume, that is, cubic feet per pound, at different temperatures at supercritical pressure. It should be noted that in the temperature range of the water wall outlet namely, between 731 and 752 at the supercritical pressure indicated that there is a rather sharp change in specific volume for a 'comparatively small change in temperature although there is only a single'phase fluid and each unit of heat added or subtracted will change the temperature of the fluid. This difierence is apparent by a comparison with the line 104 which-shows the characteristics of the fiuid at subcritical pressure and shows that in the region indicated by reference numeral 106 the specific volume changes without any change in temperature andthat the fluid is a two phase fluid with the water and the steam at different densities existing at the same temperature.

,On the curve of FIGURE 5 the line 108 indicates a system resistance curve, which is an indication of the flow in gallons per minute at a selected water wall outlet temperature and at a selected percentage of feed water flow in the parallel recirculating system. The line 108 indicates the head produced between the mixing tank and the water wall outlet header 18 by a flow of a selected number of gallons per minute through the recirculating piping 60 at a temperature of 750F. when the feedwater pump is delivering 60% of its fullcapacity. The position on the curve marked by the reference numeral indicates the volume in gallons per minute in the recirculating system at 750 F. which will produce a flow of three feet per second at the water wall inlet. This point also indicates the head between the mixing chamber 20 and the header 18 which would be produced incident to this recirculating flow under the conditions mentioned. Similar system curves are shown for a 5% flow, curve 112 and a 30% flow curve "114 at-temperatures of 730 and740 respectively which are the temperatures indicated by the graph of FIGURE 4 as the temperatures which can 'be expected atthe water wall outlet under the 5% and the 30% load conditions. A pump system curve 116 shows the pump characteristics after the pressure drop in line 60 has been subtracted from the pump characteristic curve 92. This providesv us with a pump system curve which will show the pump characteristics between 'the mixing chamber 20 and the outlet header '18. The intersection of the recirculating system curves 108 and 112and 114 with the pump system curve 116 will establish the pump performance characteristics when connected into the recirculating system and delivering recirculating fluid to the heating tubes 12 and 14. With the intersection points 118, 120 and 122 established, curves such as those shown in FIGURE 6 and FIGURE 8 may be drawn showing the pump performance in gallons per minute in FIGURE 6 and in head of fluid pumped in FIGURE 8 for the whole range of the feed water flow. The curve 126 of FIGURE-6 showing the required flow in the recirculating system for three feet per second flow at the water wall inlet may be calculated, or plotted from the three feet per second points such as 110, 120 and 124 of FIGURE 5. The curve 128 may be plotted from the intersection points of 118, 120 and 122 of FIGURE 5. In a similar manner in FIGURE 8 the three feet per second curve 130 may be plotted from the three feet per second points in FIG- URE 5 or may be calculated, and the curve 132 indicating the head developed between the mixing tank 20 and the outlet header 18 by the feed water pump might also be calculated or determined from the selected feed water and temperature conditions. The pump performance curve 134 may be picked off from the intersection points 118, 120 and 122 of FIGURE 5. Both of these curves, FIGURE 6 and FIGURE 8, show that the uncontrolled self-regulating recirculating pump which has the characteristics of a reduced flow when the head pumped against is increased will produce the required three feet per second flow at the water wall inlet at about 30% load and will automatically cease delivering any recirculating flow when the feed water flow reaches approximately 88% of full load. FIGURES 6 and 8 also show that the water wall flow area in a once-through system can be made large enough so that the feed water pump will not produce the three feet per second flow at the water wall inlet until it has attained about 70% of capacity and that the uncontrolled recirculating pump will maintain that three feet per second flow automatically under all conditions and will even increase the flow rate at the higher firing rates accompanying the increased feed water flow which is a preferred condition.

The performance of the uncontrolled pump can be illustrated in another manner by plotting the water wall inlet velocity against the percent of the feed water flow as shown in FIGURE 7. The velocity may be figured from the recirculation flow in gallons per minute at points 118, 120 and 122 of FIGURE 5 corrected for the temperature at the water Wall inlet and added to the feed water flow. Line 136 shows the three feet per second line below which the velocity at the water wall inlet should not drop. 'Line 138 shows the velocity produced by the feed water pump without assistance from the recirculating pump and line 140 shows the velocities obtained with both the feed water and the recirculating I once-through flow only.

. center wall.

9 pumps operating and with the recirculating pump handling water at approximately the temperature indicated at the water wall outlet of FIGURE 4. All three curves,

- FIGURES 6, 7 and 8 and particularly FIGURES 7 and 8, show that the recirculating pump may be stopped at 70% load and from there on to the higher loads the feed pump alone'will maintain the required safe flow at the water wall inlet. However, the recirculating pump may be allowed to continue to operate and will automatically cease delivering at about 88%. This emphasizes another safety feature, concerning the temperature fluctuation at the inlet of the hot fluid, emanating from the pump, into the nnxing vessel 20.

When the recirculating unit is operating belowthe load at which the pressure differential across the water wall equals the head developed by the pump, the hot fluid'from the pump maintains the pipe and nozzle into the mixing vessel at the temperature of the hot fluid. For a further description of a mixing vessel reference may be made to my application for Method and Apparatus for'Mixing Together Fluids, Serial No. 112,084,

filed May 23, 19 61 and assigned to the same assignee.

When the load has increased to the point where the pump head is unable to overcome the pressure differential across the furnace walls the flow of hot fluid into the mixing vessel stops and the check valve 88 downstream of the circulating pump closes. Piping and nozzle will then be subject to turbulent back Wash from either the mixed fluid in the mixing vessel or possibly from the cold fluid coining through pipe 46 from the economizer.

If the steam generator operates at a fluctuating load of or l% or so around the no delivery point of the pump then the temperatures of the nozzle will flucmate in a similar manner between the hot fluid temperature and the mixed temperature in the vessel as the pump alternately delivers and then stops delivering. This is an undesirable condition because the temperature fluctuation thus produced in heavy Wall piping can lead in time to fatigue failure.

With this no-delivery point of the pump at 88% load as shown in this application, and with the water wall velocity considerably in excess of the required minimum, the operator may shut down the circulating pump, if the load as reflected in the percentage of feed water flow starts fluctuating about this point, and particularly if the fluctuation is expected to continue. In shutting down .the pump the check valve 88 will then close and the piping and nozzle and the mixingvessel will be cooled to the least temperature and remain there while the feed water pump will supply all of the flow necessary to maintan the three feet per second at the water wall inlet.

This is a marked improvement over the structure shown in the British Patent 768,201 where the recirculating pump has a maximum head limited to coincide with the achievement of minimum velocity in the Water wall by When fluctuation takes place about that point, shutting down of the pump would endanger the water wall during the downward fluctuation of the load and continued operation of the pump will produce the above described temperature fluctuation and thereby subject the nozzle connection to fatigue and possible failure.

While the structure described above is a satisfactory structure modifications can be made by moving the pump 7 from its parallel position'in line 66 as shown in FIG- URE 2 to the series position shown in FIGURE 9'in whichthe pump 210 is in series with the fluid pump 22 andconstitutes part of the line 290 connecting the mixing tank 20 with the header 24 at the bottom of the Check valve 88 is allowed to remain in the line 60 to prevent short circuiting of the feed water from the feed pump 22 around the center and water Walls 12 and 14. Acheck valve 202 is placed in the conduit forming line 200 and a branch 204 leadingto the intake of the pump 210 is connected to the line 200 1G upstream of the check valve 2il2 between the valve 202 and the mixing tank 20 and a pump discharge line 206 connects the pump discharge with the line 200 downstream of the check valve 202 between the valve 202 and the header 24. If desired a shut-oi? valve 212 can be placed in the line 204 and a shut-off valve 214 can be placed in the line 206 to isolate the pump 210 from line 200. The check valve 202 will prevent short circuiting of the pump 210 and will permit feedwater flow to bypass pump 210 and thus prevent excessive economizer pressure if either or both valves 212 and 214 are inadvertently closed. The connections 206 and 200 between the recirculating pump'outlet and the header 24 connected to the bottom of the heating tubes is uncontrolled and has a passageway of normally fixed dimensions. The valve 214 is not a control valve but is an isolating valve for use in isolating the pump for repairs and renewal. As with the device shown in FIG. 2, the

recirculation system conduit or passageway 60 including the conduits 2%, 204, and 2% may contain check valve 83 and isolating valves 212 and 214 but is otherwise of fixed or generally constant fluid flow area and .thus will provide a recirculation passageway or conduit it becomes progressively cooler as the load increases while the water from the water wall outlet becomes progressively hotter. The pump 216 being in series with the feed pump must now handle not only the recirculating water but also all of the feed water as well. However, because of the dilference in temperature of the water being handled the total capacity of the pump and the head of the pump as measured does not have to be as large as the recirculating pump required for pumping the hot fluid in line 69 as shown in FIGURE 2. The pump 210 will create suflicient head between the water wall outlet 18 and the center wall inlet header 24 to force recirculating water through the center walls and water walls and thefeed water pump 22 will supply enough additional water to satisfy the load requirements of the turbine. It will be understood that if desired the pump 216 can be placed directly in the line 2% andthe valve 2612 closed or omitted. The pump 210 as shown in FIGURE 9 with the valve 2tl2 and its connections omitted will be designed to have enough free flow space through the centrifugal pump to permit passage of all or part of the feed water through the pump 210 with the pump shut down without material pressure drop through the idle pump.

The pump characteristic curve of pump 210 is shown in FIGURE 10 and the system characteristic curve with 5, 30 and 60% of the feed water flow are shown intersecting the pump characteristic curve. In FIGURE 10, as the pump is not an integral part of the recirculating piping the pressure drop can not be readily applied to the pump curve as it is in FIGURE 5. In FIGURE 10 the system resistance curves are first established without the recirculating piping and then corrected for theh ead lost in the recirculating piping. As before, where the pump curve 216 intersects the system curve 218, representing the system characteristics for 5% of feed water flow, will establish a point indicating the gallons per minute and the head of mixed water being pumped by the circulating pump 210 under the temperature and head conditions for 5% of feed water flow. Similarly, the intersection of the line 216 withthe line 229 indicating the system characteristics for 30%:of flow will establish the pump head and volume characteristics under the conditions of 30% of feed water flow and similarly, the inter- 1 1 section of the 'line 216 with the line 222 showing the system characteristics at'60% flow will establish a point indicating the head and volume characteristics of the pump at the 60% feed water flow condition.

With these points we can now add line 224 to FIG- URE 6 which will'show the gallons per minute of recirculating flow and show how it compares with the recirculating 'flOW required to maintain the necessary three feet per second at the water wall inlet. The gallons per minute indicated by line 224 arethe gallons indicated by the intersectionpoints on FIGURE 10 minus-the quantity of 'feed water flow.

Similarly, by correcting the gallons per minute of FIGURE 10 or the gallons per minute of line 224 of FIGURE 6 to the temperature at the water wall inlet and dividing by the fixed area of the water wall passages the curve 226 can be added to FIGURE 7 to show the velocity that will be maintained at the water wall inlet by the constant speed free running pump 210.

From these curves it will be seen that the pump 210 establishes the minimum safe flow of three feet per second at the start up feed water flow of say about and will maintain a slight excess over the required three feet per second up to approximately 80% of the feed water flow at which point the feed water establishes a head across the center and water walls sufficiently high to block any further pumping of recirculating fluid by the pump 210. Above this point the pump 210 may be shut down and the feed water fiow allowed-to flow through the pump or through the by-pass around the idle pump 210 and through the check valve 202 if such a check valve is utilized. As indicated above, if the load is fluctuating about the 80% point the pump 210 may be shut down to avoid the-thermal stresses in the nozzles of the mixing tank 20 and the necessary flow of three feet per second will be maintained by the feed water pump.

FIGURE 11 is another chart showing the characteristics of the system by plotting the head in feet or" the mixed fluid pumped against the percentage of feed water flow. In this graph it will be noted that as the percentage of feed water increases the head as measured in the feet of Water pumped by pump 21 0 decreases to maintain the three feet per second flow. This is because the quantity of cool feed water is increasing and the quantity of hot recirculated water is decreasing so that the mixed fluid has a decreasing temperature and is consequently heavier and will require a smaller number of feet of that fluid to produce the same pressure, or pounds per square inch. It should also be noted that although the head in feet of fluid pumped by the pump 210 as indicated by line 226 also decreases it does not decrease as rapidly as the required head as indicated by line 223 so that a safe margin is maintained above the required head, as the feed water flow and the corresponding firing rate increases giving rise to more severe conditions, in which the slight increase in' flow will provide an additional safety feature at the higher firing rates.

It will thus be seen that this second embodiment, utilizing the constant speed free running pump floating on the through-flow system and located in the line 205 in series with the feed pump, presents characteristics differing somewhat from those of the FIGURE '2 embodiment in which the recirculating pump is arranged in parallel with the feed pump but retains the very important features of being self-regulating Without any outside control mechanism and without any of the mechanism for limiting the pump flow except its inherent self-limiting features.

Still another position in which the pump could be placed and provide satisfactory performance and regulation would be to place the pump 21h 'in series with the feed pump by placing it in the downcon'ier 28' as shown in FIGURE 12 between the center wall outlet header 26 and the water wall inlet header 30. This would be a construction similar to that shown in FIGURE 9 if the center wall and its headers were omitted and the line 200 connected directly to the water wall inlet header 3t). It is believed unnecessary to draw curves showing this embodiment for they would be similar to those shown in FIGURES 6, 7 and -1-1 for the structure of FIGURE 9 except for a temperature correction incident to the heat added in the center wall.

Stillanother position in which the recirculating pump may be placed is in the through-flow line 300, as shown in FIGURE 13, at the outlet of the water Walls 14. Like the structure shown in FIGURES 9 and 12 this modification shows the free running recirculating pump in series with the feed pump and pumping the mixed flow and providing the head for recirculating flow and providing mixed flow of at least the critical velocity in the heating tubes at loads below the cut-off point of the recirculating pump.

Free-running as used in this specification and claims is limited to mean running free without separate control mechanism responsive to any parameter of the fluid pumped for controlling either the pump, pump inlet means, pump outlet means or the motor. Hence the freerunning pump in the specific embodiment disclosed would be a pump driven by a motor connected to a power source, such as the usual alternating current electric power line having a substantially constant voltage and frequency, hence running at substantially constant speed without motor performance control or regulating mechanism between the motor and the source and with the pump operating free of controls in the pump circuit.

Floating as used in this specification and claims is limited to means connected across or in a line, passageway or circuit without controlling mechanism in the connec. tion for limiting or controlling the inlet or discharge flow of thepump, so that the full pump pressure is applied across or in the line whenever the pressure across or in the line would otherwise fall below the pump pressure. Hence with the recirculating pump floating on or in a selected portion of the through-flow line the pump output is self-regulating and uncontrolled by control mechanism and dependent only on the pump characteristics and the line conditions.

It is to be understood that the invention is not limited to the specific embodiment herein illustrated and described but may be used in other ways without departure from its spirit and that various changes can be made which would come within the scope -of the invention which is limited only by the appended claims.

What is claimed is:

1. A forced flow once-through steam generator having means for supplying hot gases and having working fluid heating tubes having an inlet, an outlet and an intermediate portion therebetween exposed to said hot gases, feed pump meansuconnected to said inlet of said tubes and supplying a positive, once-through flow of Working fluid to the heating tubes at super-critical pressure, recirculating means including a passageway having generally constant recirculating fluid flow area during normal recirculation fluid flow throughout a range of operation of the generator connecting the outlet to the inlet of the fluid heating tubes and including a free running, self regulating recirculating pump having the inherent characteristic of decreasing pumped volume with increasing pumped head and establishing a positive recirculation of the working fluid through the heating tubes and the recirculation means in addition to the once-through flow.

2. A steam generator as defined in claim 1 in which said heating tubes require a predetermined minimum volume flow rate for safe operation, and said recirculating pump has a self regulating flow rate automatically decreasing with increas'eof feed pump flow rate, said recirculating pump having a capacity such that the combined flow rate of said pumps is substantially equal to said safe flow rate at the lowest combined flow rate and'increases with increase in'feed pump flow rate and decrease in rethe feed pump means supplies working fluid to said tubes at a rate corresponding to the demand for the heated working fluid from said tubes, the pressure drop across said tubes and said recirculating pump increasing with increase in feed pump'means' flow rate, said recirculating pump having a flow rate automatically, decreasing with increase in pressure across said tubes, to automatically vary the combined flow rate through said tubes, said tubes requiring a predetermined volume flow rate for safe operation, said combined flow rate being at least equal to said safe flow rate at all times, and said recirculating pump having and applying to said tubes a pressure head at zero delivery rate greater than the pressure head required to maintain said predetermined flow rate, without recirculation.

4. In a furnace having walls formed by a plurality of tubes and having means for heating a working medium in said tubes for supplying avarying working medium demand, said tubes having an inlet and an outlet and requiring a predetermined velocity flow rate of said medium in said tubes for safe operation, feed water pump means connected with said inlet and supplying said medium to said tubes at supercritical pressure and at a rate to satisfy said demand whichrate, at times, is insufflcient to produce said predetermined velocity in said tubes,

the improvement comprising a free running recirculating pump having the inherent characteristic of decreasing pumped volume with increasing pumped head, means connecting said recirculating pump in fluid flow relationship directly across said tubes to recirculate the working medium through said tubes in addition to the medium pumped by said feed pump to thereby maintain said predetermined safe velocity flow rate. r

, 5. A combination as claimed in claim 4 in which the recirculating pump is connected directly across said tubes in series with said feed pump.

6. A combination as claimed in claim 4 in which said recirculating pump is a pump having an output varying inversely with pumped head and having a minimum head under operating conditions sufficient to maintain said predetermined velocity flow rate at said times when the through flow through the tubes is at a rate less than said predetermined rate and having a no recirculating flow delivery head greater than the feed pump means head required to maintain said flow rate alone.

7. In a forced-flow once-through vapor generator means for supplying hot gases and having working fluid heating tubes having an inlet, an outlet and an intermediate portion between said inlet and outlet exposed to said heated gases, a mixing chamber, feed pump means supplyingthrough-flow working fluid first to said chamber and then to said inlet of said tubes at a rate to satisfy the vapordemand, recirculation means comprising a recirculation passageway including said mixing chamber, a connection from said chamber to the inlet of said tubes, said tubes, and an outlet connection connecting the outlet er said tubes with said-chamber, the improvement comprising a free-running recirculating pump in said passage- Way, said passageway includingmeans connecting said recirculating pump in fluid flow relation across said tubes and conducting working fluid from said outlet to the inlet of said recirculating pump, said pump forcing working fluid through said tubes in addition to said through-flow fluid to provide a combined flow of working fluid, means preventing flow from said chamber to said outlet in said outlet connection, said free-running pump having the inherent self-regulating characteristic ofautomatically varying the quantity of fluid pumped inversely with the varia-.

tion of the head across saidtubes to automatically vary the combined flow and thereby maintain at least a preselected minimum velocity of working fluid in said tubes. 8. A system as claimed in claim 7 in which the rerecirculating means including means connecting the outlet of said tubes with the inlet of said tubes and also including a substantially constant speed recirculating pump having the inherent characteristic of pumped flow varying inversely'with pumped head floating on the through flow line' for establishing a positive recirculation flow of the working fluid through 'said heating tubes and the recirculating means.

. 10. .A steam generator as claimed in claim 9 'in which said recirculating pump has a no recirculating flow delivery pressure materially greater than the pressure across said tubes created by said feed pump means forcing fluid through said tubes at said critical velocity.

11. A generator as claimed in claim 9 in which the recirculating pump is in the through-flow passageway in series with said feed pump means.

12. A generator as claimed in claim 11 including a mixing chamber connected with the inlet of said tubes and receiving both the once-through and the recirculating flow and in which the recirculating pump is located between the chamber and said inlet.

13. A generator as claimed in claim '11 including'a by-pass around said recirculating pump and a check valve in said. by-pass preventing short-circuiting of said recirculating pump.

14. A forced flow once-through steam generator for supplying a variable steam demand and having a once-through flowpassageway including vertically arranged parallel flow working fluid heating tubes having an inlet, feed water pump means connected with said inlet of said tubes for establishing the working pressure and a positive oncethrough flow of working fluid to said tubes at supercritical pressure and at said demand rate, said through flow passing through said tubes creating a pressure drop across said tubes increasing with said demand, recircu-' lat1ng mean comprising a recirculating pump floating on the once-through passageway and connected in fluid flow relation across said tubes by conduit means unobstructed by variable control mechanism during recirculation operation, means driving said pump at a substantially constant speed, said recirculating pump pumping against said pressure drop and having theinherent characteristic of reducing volume pumped with increase in head across said tubes and having sufiicient capacity to provide with said through flow, a preselected critical velocity of working fluid in said generator at a selected steam demand substantially less than that necessary to provide'a critical velocity of through flow alone and provide a higher velocity athigher steam demand.

15. A forced flow once-through steam generator for supplying a varying steam demand and having means for creating hot gases and having working fluid heating tubes having an inlet, an outlet and an intermediate portion between said inlet and said outlet exposed to hot gases and in which the temperature of fluid discharged from said tubes increases with increased steam demand, through-flow means including said tubes and feed pump means connected to said inlet and supplying through flow fluid cooler than said discharged fluid for supplying a positive once-through flow of working fluid at supercritical pressure and in'accordance with'said demand, said tubes requiring a predetermined velocity flow rate of said working fluid for safe operation, recirculation means connected to the said inlet and said outlet of said fluid heating tubes and including a substantially constant speed recirculating pump, having an output inherently varying inversely with pumped pressure, floating on the throughflow means for establishing a positive recirculation of the working fluid through the heating tubes, means combining said through flow and the recirculation flow to provide a combined flow of at least said predetermined velocity, the temperature of recirculation fluid discharged from said tubes for combination with said through-flow, and the feed quantity of said cooler through flow, increasing with increased steam demand, the quantity of increased temperature recirculation fluid reducing with increased steam demand and when combined with the increased quantity of cooler through flow producing a temperature of the combined fluid decreasing with increased steam demand and a reduction in the volume of the recirculation fluid in the combined fluid, said recirculating .pump located between said combining means and the inlet of said tubes and pumping said reduced volume recirculating fluid, said recirculating pump recirculation output automatically decreasing with increased through flow to a no-delivery recirculation output at a steam demand and through flow greater than the demand at which the through-flow alone will provide a safe velocity flow.

16. A forced flow once through vapor generator having means for generating hot gases and having-a through flow passageway including working fluid heating tubes having an inlet, an outlet and an intermediate portion,

between said inlet and said outlet exposed to said hot gases, feed pump means connected to said inlet establishing the Working pressure and a positive once through flow of working fluid through the passageway, a check valve in said passageway preventing reverse flow in said passageway, recirculating means including means connecting the outlet of said tubes with the inlet of said tubes and also including a recirculating pump having an inlet and located in said passageway in series assisting relation with said feed pump and means connecting said pump across said check valve with the inlet of said recirculating pump connected upstream of said check valve, and an isolation valve on each side of said recirculation pump in said pump'connecting means.

17. In combination with a forced-flow once-through steam generator for supplying a variable steam demand and having vertically arranged working fluid heating tubes having an inlet and an outlet, feed water pump 'istic of decreasing recirculation flow less than the inmeans, means connecting said pump means to the inlet of said tubes, for establishing the Working pressure and supplying a positive once-through upward flow of working fluid to said tubes at supercritical pressure and at the demand rate, said connecting means including a mixing vessel at an elevation substantially equal to the top of said tubes, connected with said inlet by a downwardly directed conduit, and receiving the feed pump discharge, a recirculating system connecting the outlet of said tubes with said inlet and including a self regulating recirculating pump connected in fluid flow relation with said mixing vessel and said tubes for establishing a positive recirculation of heated fluid through said tubes and mixing vessel, said recirculating pump floating in the connecting means between the mixing vesseland the inlet of said tubes, the pressure differential between the comparatively cool fluid in said conduit and the heated fluid in said tubes assisting said recirculation.

18. In the method of operating, at supercritical pressure and varying loads, a forced flow once-through steam generator having a safe minimum velocity of the working fluid and in which the once-through flow varies with the load and hot Working fluid is recirculated, through a portion of the generator, to provide said safe velocity at loads below that at which the through flow alone provides said safe velocity, by a pump, floating on the oncethrough circuit through said portion, and having a no delivery condition at a load above that at which the through flow alone provides the minimum safe velocity of the working fluid, the steps of recirculating hot fluid at loads above that at which the once-through flow alone provides the minimumsafe velocity, and'in the event of a fluctuating load fluctuating both sides of said no delivery condition, discontinuing recirculation and operating with the through flow alone.

19. A method of operating a forced flow once-through steam generator for supplying a varying steam demand and having means providing hot gases and having working fluid heating tubes exposed to said hot gases and requiring a predetermined safe velocity flow of working" fluid in said tubes, feed supply means connected with said tubes for torcing through flow through said tubes and having a through flow system including said tubes and a recirculating system in parallel with said through flow system and including said tubes and a recirculating pump floating on the through flow system having a recirculating output varying inversely with the pressure drop across said tubes, comprising the steps of forcing a positive once-through flow of Working fluid through said tubes at supercritical pressure, said through flow being insuflicient to produce said safe velocity below a predetermined steam demand, forcing a recirculating flow of heated working fluid through said tubes, increasing through flow and the pressure differential across said tubes with increasing steam demand, applying said increased pressure diiferential across said pump to thereby cause a decrease in recirculation flow with the charactercrease in through flow through said tubes which caused the increase in pressure diflerential, combining said through flow fluid and said recirculating flow fluid to bring the flow velocity of the combined fluid in said tubes to a predetermined safe velocity below said predetermined steamdemand, and to produce a flow velocity of combined fluid increasing from said safevelocity to higher velocities above s'aid'safe velocity with increasing steam demands.

20. A method of operating a once-through vapor generator supplying a variable demand and having working fluid heating tubes and a recirculating system recirculating fluid through said tubes and recirculating means conmeeting the outlet with the inlet of said tubes and having in said connecting means an upstream portion and a downstream portion, the step of withdrawing fluid from said tubes through said upstream portion at a reduced pressure and supplying 'fluid to said tubes through said downstream .portion at an increased pressure to recirculate fluid through said tubes, supplying a through flow of working fluid through said tubes in accordance with said demand, mixing said through flow 'fluid with said reduced pressure recirculating fluid while supplying the through flow fluid at a rate which will maintain the pressure at the mixing point of said through flow and recirculating fluids below the pressure at said tube outlet, increasing the rate of through flow supply to increase the pressure at said mixing point to a pressure greater than the pressure at said outlet, to block flow of recirculating fluid from said outlet to said mixing point and thereafter maintaining a safe rate of flow through said tubes by said through flow alone.

References Cited in the file of this patent UNITED STATES PATENTS 2,088,623 Thompson Aug. 3, 1937 3,038,453 Armacost June 12, 1962 FOREIGN PATENTS 719,753 Great Britain Dec. 8, 1954 818,159 Great Britain u Aug. 1 1959 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 5,135,252 June 2, 1964 Willburt W. Schroedter or appears in the above numbered pat- It is hereby certified that err said Letters Patent should read as ent requiring correction and that the corrected below.

Column 12, line 32, for "means" read mean line 35, for "controlling" read control Signed and sealed this. 13th day of October 1964.

SEAL A ttest:

EDWARD J. BRENNER ERNEST W. SWIDER I Attesting Officer Commissioner of Patents 

1. A FORCED FLOW ONCE-THROUGH STEAM GENERATOR HAVING MEANS FOR SUPPLYING HOT GASES AND HAVING WORKING FLUID HEATING TUBES HAVING AN INLET, AN OUTLET AND AN INTERMEDIATE PORTION THEREBETWEEN EXPOSED TO SAID HOT GASES, FEED PUMP MEANS CONNECTED TO SAID INLET OF SAID TUBES AND SUPPLYING A POSITIVE, ONCE-THROUGH FLOW OF WORKING FLUID TO THE HEATING TUBES AT SUPER-CRITICAL PRESSURE, RECIRCULATING MEANS INCLUDING A PASSAGEWAY HAVING GENERALLY CONSTANT RECIRCULATING FLUID FLOW AREA DURING NORMAL RECIRCULATION FLUID FLOW THROUGHOUT A RANGE OF OPERATION OF THE GENERATOR CONNECTING THE OUTLET TO THE INLET OF THE FLUID HEATING TUBES AND INCLUDING A FREE RUNNING, SELF REGULATING RECIRCULATING PUMP HAVING THE INHERENT CHARACTERISTIC OF DECREASING PUMPED VOLUME WITH INCREASING PUMPED HEAD AND ESTABLISHING A POSITIVE RECIRCULATION OF THE WORKING FLUID THROUGH THE HEATING TUBES AND THE RECIRCULATION MEANS IN ADDITION TO THE ONCE-THROUGH FLOW. 